GUATEMALA CITY, AUG. 8 -- A historic Central American peace plan signed here yesterday carries great potential for ending years of war in the region, but its implementation will require hard negotiating in the months to come with the attendant risk of a breakdown, according to analysts here.

If these negotiations succeed, the plan may bring peace and economic development to a region racked by military coups, death squads, insurgencies and poverty, according to analysts here. They also say it could bring a peace prize to the plan's principal author and motivator, Costa Rican President Oscar Arias.

If the plan fails, however, the result is likely to be further polarization, a deepening of the region's economic morass and a continuation of the bloodletting.

The major concern, now emerging in the wake of yesterday's euphoria, is that different interpretations of the agreement's major provisions could lead to impasses in the negotiations that lie ahead.

Also key to the plan's success is the acceptance of it by the United States, which is called upon to stop supplying aid to the Nicaraguan rebels in 90 days, simultaneously with the implementation of other provisions that would "democratize" Nicaragua.

{President Reagan said Saturday that the United States will be "as helpful as possible" in working for peace in Central America, but he stopped short of endorsing the peace plan and said it must take into account "the interests of the Nicaraguan resistance," The Associated Press reported.

{"I welcome this commitment to peace and democracy by the five Central American presidents, and I hope it will lead to peace in Central America and democracy in Central America," Reagan said in a statement released by the White House.

{Referring to the U.S.-backed Nicaraguan rebels, known as counterrevolutionaries or contras, Nicaraguan President Daniel Ortega said in a speech after returning from the summit that the Sandinista military would continue "to fulfill our defensive plans . . . to keep annihilating the mercenary forces," AP reported.

{He added, however, that Nicaragua "must establish a mechanism for a cease-fire along the lines that those who are armed and are in the counterrevolution have the chance to give up their arms and enjoy all the guarantees of their security and life and civil rights."}

The agreement was signed yesterday at the end of a two-day Central American summit by presidents Ortega and Arias, Vinicio Cerezo of Guatemala, Jose Napoleon Duarte of El Salvador and Jose Azcona of Honduras. According to Central American and U.S. analysts, the agreement represented significant concessions by Ortega and Duarte, whose countries are most affected by rebellion, economic dislocation and deepening poverty.

"For whatever reason, the Sandinistas were much more eager for an agreement and open to negotiations than they were last year," said William Goodfellow, director of the Washington-based Center for International Policy, a nonprofit research organization affiliated with the Fund for Peace.

Goodfellow said Nicaragua's deteriorating economy, the strengthening in the past 10 months of the contras and the influence of pragmatists in the Sandinista government have combined to convince leaders there that they need an accord with the United States.

"The Sandinistas know that if they are not flexible, the war will go on," Arias said last night in an interview after the agreement was signed. "The Nicaraguan economy is a mess, and they realize it's a mess."

Another factor that may have figured in the Sandinista thinking, analysts here suggested, was Managua's growing dependence on aid from the Soviet Union, which is engaged in delicate arms control negotiations with the United States and has indicated some reluctance to go on bankrolling Nicaragua at current high levels.

In signing the plan, Ortega pledged his leftist government to democratic reforms including freedom of the press and political activities, repeal of a state of emergency and free elections monitored by international observers. Elections are to be held next year by all five countries for a Central American parliament, and local and presidential elections are to be held according to each country's existing timetable.

Ortega initially had ruled out internal "democratization" measures, arguing in a news conference before the summit opened that democracy was already in place in Nicaragua and that, in any case, such "internal" matters were not a proper subject for international talks.

The democratization provisions are to start taking effect in 90 days simultaneously with cease-fires in the region's wars, general amnesties, an end to military and certain other types of aid to insurgencies and prohibitions on the use of territory in one state to attack another.

In signing the agreement, the five presidents put aside a new Reagan administration peace plan that initially had thrown the meeting here into confusion and threatened to overshadow the Costa Rican plan under discussion.

Julio Adolfo Rey Prendes, El Salvador's communications minister, said the Reagan proposal caused "indisputable worry" that the Arias plan would be shelved and that the Sandinistas might use this as a pretext to withdraw from the talks. The effect, he said, was to spur the Central American leaders to come to an agreement.

"There's no question that the Reagan plan added a sense of urgency to the summit," Goodfellow said. "It was like a sword of Damocles hanging over their head."

One of the more difficult aspects of the plan to be negotiated is expected to be on a cease-fire between the Sandinistas and the contras. The contras say they now have 17,000 fighters in Nicaragua battling the estimated 75,000-member Sandinista Army. Contra leaders who gathered here during the summit said they insisted on direct involvement in any cease-fire talks and demanded a cease-fire in place with provisions for delivery of "humanitarian aid," such as food and medicine, to their troops in Nicaragua.

The Sandinistas' reaction to these demands was not immediately known, but in the past they have refused to negotiate with the contras. The peace plan does not explicitly require the Sandinistas to negotiate directly with the contras, although the rebels are interpreting the plan as implying negotiations. It is also unclear how the Salvadoran rebels and guerrillas in Guatemala will react to the plan. The peace plan has focused attention mainly on the Nicaraguan civil war, but the plan covers all five Central American countries and its war-ending provisions also apply to El Salvador and Guatemala.

An estimated 4,000 to 6,000 guerrillas of the Marxist Farabundo Marti National Liberation Front are fighting El Salvador's 56,000-member armed forces in a war that began eight years ago. The Salvadoran situation is complicated by extreme rightist groups, which also may reject the plan on grounds that it implicitly recognizes the guerrillas. The prospect then arises that right-wing death squads, active early in the war, could re-emerge and effectively scuttle a reconciliation, analysts said.

In Guatemala, Marxist guerrillas now allied under the Guatemalan National Revolutionary Union have dwindled to fewer than 2,000 in an insurgency that began 25 years ago.

Guatemalan forces have used brutal tactics to supress the rebellion, killing thousands of suspected rebel sympathizers. Yet, the rebels persist and have stepped up their efforts through a clandestine radio station that began operating in May.

The first test of the agreement is expected to come with the formation of "commissions of national reconciliation" to verify compliance with amnesty, cease-fire, democratization and other provisions. The commissions are to be formed within 20 days of the signing through a complicated process designed to ensure participation by representatives of governments, their opponents and neutral individuals.

"Working groups" to be named by the five Central American foreign ministers are to meet within 15 days and carry out the commitments regarding the various provisions. Arias said separate groups would be formed to deal with democratization, cease-fires, amnesties, aid suspension and other matters.

{After the signing of the peace plan, Nicaragua announced it would withdraw a World Court case against Costa Rica as a gesture of good will, Reuter reported. Nicaragua had accused Costa Rica and Honduras of allowing the contras to use their territories as bases of operations. Managua also may consider withdrawing its charges against Honduras.}